1. Technical Field
Aspects of the present invention relate to an image scanning device for scanning an image on a document.
2. Related Art
An image scanning device of the so-called “flat bed” type is generally equipped with an image sensor such as a CIS (Contact Image Sensor) moving to and fro along the platen glass. When a document is placed on the platen glass of such an image scanning device and an instruction for starting the scanning of the document is inputted by the user, the image on the document is optically scanned by the CIS during the movement of the CIS. The scanned image of the document is outputted from the CIS as an image signal (electric signal).
Incidentally, the scanning of a thick document (e.g. book) is usually carried out with the document cover of the image scanning device open, and thus external light can enter the document table (in which the CIS is installed) through the platen glass during the scanning of a thick document. In such document scanning with external light entering the document table, the CIS receives not only reflected light from the document but also the entering external light. Since the signal level of the image signal outputted by the CIS becomes higher compared to cases where no external light enters the document table, the scanned image outputted by the CIS becomes a whity image.
In order to resolve the above problem, a digital copier capable of correcting an image signal deteriorated by the entrance of external light has been devised, as disclosed in Japanese Patent Provisional Publication No. 2002-185796 (hereinafter referred to as JP2002-185796A), for example. The digital copier disclosed in JP2002-185796A is equipped with a light amount sensor which is placed outside the platen glass (the top of the document table to be covered by the document cover). In this digital copier, the light amount of the external light (which is substantially proportional to the degree of opening of the document cover) is detected by the light amount sensor and conditions of image processing executed to the scanned image signal are changed depending on the light amount detected by the light amount sensor, by which the correction of the output image based on the light amount detected by the light amount sensor is carried out.
Another configuration described in JP2002-185796A employs an extending part of an elongated CCD (extending in the main scanning direction to the outside of the document scanning area) in place of the light amount sensor. With this configuration, the light amount of the external light can be detected without the need of providing a sensor specifically for the detection of the external light.
However, the image correction employed by the digital copier of JP2002-185796A involves the following problem. When a document that does not let through light (book, booklet, etc.) is placed on the platen glass, the external light enters the document table through areas of the platen glass at both edges of the document in the main scanning direction. In this case, within the whole image signal obtained by the document scanning, only parts of the signal corresponding to both edges of the document in the main scanning direction are affected by the external light. On the other hand, when a document that lets through light (thin paper, transparent document, etc.) is placed on the platen glass, the external light enters the document table uniformly through all parts of the document. In this case, the whole image signal obtained by the document scanning is affected by the external light. The digital copier of JP2002-185796A, making the same image correction irrespective of the type of the document (document type) is incapable of carrying out optimum image correction for the removal of the effect of external light from the image signal obtained by the document scanning.